The brain is composed of neurons and other cell types in connected networks that process sensory input, generate motor commands, and control all other behavioral and cognitive functions. Neurons communicate primarily through electrochemical pulses that transmit signals between connected cells within and between brain areas. Noninvasive neuromodulation technologies that affect neuronal activity can modulate the pattern of neural activity and may cause altered behavior, cognitive states, perception, and motor output without requiring an invasive procedure.
Non-invasive neuromodulation includes the broad category of “transdermal electrical stimulation,” which generally refers to electrical stimulation of the nervous system (brain, cranial nerves, peripheral nerves, etc.) through a subject's skin. Specific examples of transdermal electric stimulation (hereinafter “TES”) may include transcranial stimulation, for example, through scalp electrodes and have been used to affect brain function in humans in the form of transcranial alternating current stimulation (hereinafter “tACS”), transcranial direct current stimulation (hereinafter “tDCS”), cranial electrotherapy stimulation (hereinafter “CES”), and transcranial random noise stimulation (hereinafter “tRNS”). Systems and methods for TES have been disclosed (see for example, Capel U.S. Pat. No. 4,646,744; Haimovich et al. U.S. Pat. No. 5,540,736; Besio et al. U.S. Pat. No. 8,190,248; Hagedorn and Thompson U.S. Pat. No. 8,239,030; Bikson et al. US patent application 2011/0144716; and Lebedev et al. US patent application 2009/0177243). tDCS systems with numerous electrodes and a high level of configurability have been disclosed (see for example Bikson et al. US Patent applications 2012/0209346, 2012/0265261, and 2012/0245653), as have portable TES systems for auto-stimulation (Brocke U.S. Pat. No. 8,554,324).
In general, TES has been shown to improve motor control and motor learning, improve memory consolidation during slow-wave sleep, regulate decision-making and risk assessment, affect sensory perception, and cause movements. TES has been used therapeutically in various clinical applications, including treatment of pain, depression, epilepsy, and tinnitus. In at least some cases of TES (e.g., tDCS) therapeutic use, more data concerning the efficacy of tDCS in treatment is needed.
Despite research done on TES neuromodulation, existing systems and methods for TES are lacking in at least some cases in their capacity to safely and robustly affect cognitive function and induce cognitive states in human subjects. The development of new TES methods, TES stimulation protocols, TES systems, and TES electrode configurations that induce substantial changes in cognitive function and/or cognitive state comfortably would be advantageous. Existing systems and methods can cause skin irritation or pain and are lacking with regard to the reliability and amount of change in cognitive state that can be achieved.
Electrotherapy for muscles and other peripheral nervous system applications (e.g. TENS & transdermal drug delivery) have used strategies to reduce pain, irritation, and tissue damage, including (1) higher frequencies of alternating current stimulation and (2) a beat frequency generally between 1 Hz and 200 Hz created from a difference frequency of two channels (anode-cathode pairs) of electrodes. Reduced side-effects (e.g. pain and irritation) are approximately linear across a wide range from ˜1 kHz to 100 kHz. Skin impedance is frequency dependent, with lower impedances at higher electrical stimulation frequencies. For interferential stimulation, a beat frequency of between 1 and 200 Hz is an advantageous frequency to avoid activating pain & muscle fibers that are perceived as irritating or painful. Power density also affects skin resistivity, with lower resistivity occurring at higher power densities. However, systems and methods for TES are lacking in terms of mitigation of pain, irritation, and tissue damage.
Typical transcranial alternating current stimulation protocols are also typically below 150 Hz (see Paulus 2011), consistent with frequencies of brain rhythms or below 640 Hz as used in tRNS protocols. Recently, Chaieb et al. used 1 kHz, 2 kHz, and 5 kHz tACS to induce neuromodulation (Chaieb L, Antal A, Paulus W. “Transcranial alternating current stimulation in the low kHz range increases motor cortex excitability.” Restor Neurol Neurosci. 2011; 29(3):167-75, incorporated fully herein by reference). Patent Application No. WO 2012/089,588 by inventors Paulus and Warschewske describes systems and methods of tACS at frequencies between 1 Hz and 50 kHz, including interferential tACS from two anode-cathode electrode pairs and pulsed tACS. However, existing tACS systems for neuromodulation are less than ideal for inducing cognitive effects robustly and comfortably.
One advantage of transcranial alternating current stimulation relative to transcranial direct current stimulation is reduced pain and irritation. However, existing tACS systems for neuromodulation are less than ideal in at least some instances, because alternating currents affect nervous system function (i.e., brain function) differently than direct currents. One advantage of pulsed transcranial direct current stimulation relative to unpulsed transcranial direct current stimulation is reduced pain and irritation. Pulsed transcranial direct current stimulation has been previously reported for peripheral use in patients but has not been used for targeting the brain transcranially. The Idrostar Iontophoresis Machine (STD Pharmaceutical Products Ltd, Hereford, England) delivers pulsed direct current stimulation (7 kHz, about 42% duty cycle) to address hyperhidrosis (excess sweating). Alternative transcranial electrical stimulation protocols that achieve desired effects on the nervous system with manageable amounts of pain and/or irritation would be advantageous.
It would generally be advantageous to provide devices and methods that allow transdermal electrical stimulation in a manner that overcomes the problems with pain and efficacy discussed above. In particular, it would be beneficial to provide TES devices and methods for modulating (e.g., inducing, enhancing, reversing, or otherwise increasing or changing) a cognitive effect and/or mental state. For example, TES stimulation protocols and electrode configurations that induce a relaxing, calming, anxiolytic, dissociated, high mental clarity, or worry-free state of mind in a subject would be advantageous for improving the subject's experiences and state of mind, as well as addressing insomnia and mitigating negative responses to stress. Similarly TES stimulation protocols and apparatuses that increase a subject's motivation, subjective (and/or physiological) energy level, or focus would be advantageous for improving a subject's productivity and providing beneficial states of mind. Described herein are methods and apparatuses (including devices and systems) and methods that may address the problems and opportunities discussed above.